Onboard control computers have become ubiquitous in motor vehicles, as safety, economy, and emissions requirements have continued to escalate, and conventional designs for reciprocating engines, friction braking systems, collision safety apparatus, and traction control devices have proven unequal to the requirements set out in law and the implicit demands of competitors' achievements. Successive generations of onboard control computers have acquired increasing data sensing and retention capability as the electronic art has advanced. Present external diagnostic and display apparatus, known to those skilled in the art as Scan Tools, are commonly limited to reporting the data acquired by the onboard control computer itself. Increasingly subtle subsystem failures in automobiles overload the ability of maintenance technicians not simply to read the faults detected and stored by the computers themselves, but to combine those readings with peripheral measurements in order to allow a technician to identify faults and decide on corrective actions with both speed and accuracy.
Accordingly, it is desirable to provide in the Scan Tool the ability to acquire and evaluate test data from sources other than the motor vehicle's onboard computer, and to combine those results with data acquired directly from the onboard computer. The present invention, by enhancing the Scan Tool's ability to collect data from external test devices via data input ports, and by merging the additional data with data previously available from the onboard computer into a single display with fully coordinated timing, presents to the technician a more complete picture of the status of the motor vehicle under test.